PPAPI: Add C++ wrapper for MessageHandler stuff.

ppapi/cpp/message_handler.h

Index: ppapi/cpp/message_handler.h
diff --git a/ppapi/cpp/message_handler.h b/ppapi/cpp/message_handler.h
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+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef PPAPI_CPP_MESSAGE_HANDLER_H_
+#define PPAPI_CPP_MESSAGE_HANDLER_H_
+
+namespace pp {
+
+/// <code>MessageHandler</code> is an abstract base class that the plugin may
+/// implement if it wants to receive messages from JavaScript on a background
+/// thread when JavaScript invokes  postMessage() or
+/// postMessageAndAwaitResponse(). See pp::Instance::RegisterMessageHandler()
+/// for usage.
+class MessageHandler {
+ public:
+  virtual ~MessageHandler() {};
+
+  /// Invoked as a result of JavaScript invoking postMessage() on the plugin's
+  /// DOM element.
+  ///
+  /// @param[in] instance An <code>InstanceHandle</code> identifying one
+  /// instance of a module.
+  /// @param[in] message_data A copy of the parameter that JavaScript provided
+  /// to postMessage().
+  virtual void HandleMessage(pp::InstanceHandle instance,
+                             const Var& message_data) = 0;
+
+  /// Invoked as a result of JavaScript invoking postMessageAndAwaitResponse()
+  /// on the plugin's DOM element.
+  ///
+  /// NOTE: JavaScript execution is blocked during the duration of this call.
+  /// Hence, the plugin should respond as quickly as possible. For this reason,
+  /// blocking completion callbacks are disallowed while handling a blocking
+  /// message.
+  ///
+  /// @param[in] instance An <code>InstanceHandle</code> identifying one
+  /// instance of a module.
+  /// @param[in] message_data A copy of the parameter that JavaScript provided
+  /// to postMessage().
+  /// @return Returns a pp::Var that is then copied to a JavaScript object
+  /// which is returned as the result of JavaScript's call of
+  /// postMessageAndAwaitResponse().
+  virtual pp::Var HandleBlockingMessage(pp::InstanceHandle instance,
+                                        const Var& message_data) = 0;
+
+  /// Invoked when this MessageHandler is no longer needed. After this, no more
+  /// calls will be made to this object.
+  ///
+  /// @param[in] instance An <code>InstanceHandle</code> identifying one
+  /// instance of a module.
+  virtual void WasUnregistered(pp::InstanceHandle instance) = 0;
+};
+
+}  // namespace pp
+
+#endif  // PPAPI_CPP_MESSAGE_HANDLER_H_